Does a carport slab need expansion joints at interior piers or the perimeter?

Question

I'm planning to pour a pad into an existing open carport / garage area that has mid-span supports resting on poured piers and a foundation wall going around the outside. I'm planning on digging down about 10-12", laying gravel and compacting, laying sand and compacting, then add a vapor barrier and metal mesh / rebar.

Is there anything I need to think about when pouring the new slab up to the edges of the foundation and around the piers? do I need to wrap the slab edges with foam insulation to give an expansion joint / allow them to slip past each other when settling occurs or is there a better solution?

The piers / foundation have been in place for at least 40 years. The slab would just be adding a new surface in the garage area.

EDIT: there will eventually be living space above the garage. I was sort of assuming I'd want the vapor barrier due to this. The space is approx 30' wide by 20' deep, and there's three garage slots, with posts supporting the joists above between each slot.

Answer 1

I assume the pier is above the finished floor. You are correct in thinking to provide the full depth expansion joint around the pier, as well as around the perimeter of the slab that will be poured against existing concrete.

If the garage area is large, in addition to the expansion joint, it is recommended to have a control joint at an approximate 10'-15' interval in each direction to minimize the potential for random crack. The control joint (1/8"-1/4" wide x 1" depth) shall be made by saw-cut within 24 hours of concrete placement. Around the pier, the diamond-shaped control joint is recommended to avoid diagonal cracks at the reentrant corners of the slab.

The last recommendation is the fresh concrete shall have at least 5% of entrained air to improve its durability against weathering, especially in the cold regions.

Answer 2

I'm not a concrete specialist, but I've seen many garages and basement slabs poured and observed the outcomes in an extreme environment with seasonal temperature swings of over 120° F. Virtually none had the need for expansion joints (compressible material) around the perimeter. Concrete shrinks as it cures, and it doesn't expand appreciably later unless subject to extreme heat. A carport offers shade, so that's unlikely. It can't really hurt, but it does leave a somewhat ugly and fuzzy gap around the edges.

That said, the interior piers are a different story. The size of a hole is reduced as the material around it shrinks. That means those openings will get tighter. I'd use a thin (1/4" or so) expansion buffer around them. Foam sill seal would be great. You can leave it high and trim it later if you like.

I'm much more concerned with control joints. Nearly all slabs crack, so you want to define where that happens for the best long-term appearance. Cutting 1/3-depth on a 10' grid is a good strategy, but you'll probably want to adjust for symmetry and uniformity.

Around the post footings I'd make extra cuts, particularly from the corners. I would try to center the intersection of your larger grid on the post, and cut a diamond similar to r13's image. I'd cut all the way (or as close as you can come) to the footing, though. Cracks will propagate all the way to those points.